Constraining planet structure and composition from stellar chemistry: trends in different stellar populations

¹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
e-mail: nuno@astro.up.pt
² Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
³ University of Zurich, Institut of computational sciences, Winterthurerstrasse 190, 8057 Zürich, Switzerland
⁴ Physikalisches Institut, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
⁵ Department of Reference Systems and Geodynamics, Royal Observatory of Belgium (ROB), Avenue Circulaire 3, 1180 Brussels, Belgium
⁶ Institute of Geological Sciences, Freie Universität Berlin, Malteserstr. 74-100, 12249 Berlin, Germany
⁷ Instituto de Astrofísica de Canarias, C/vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
⁸ Universidad de La Laguna, Dept. Astrofísica, 38206 La Laguna, Tenerife, Spain

Received: 13 June 2017
Accepted: 27 October 2017

Abstract

Context. The chemical composition of stars that have orbiting planets provides important clues about the frequency, architecture, and composition of exoplanet systems.

Aims. We explore the possibility that stars from different galactic populations that have different intrinsic abundance ratios may produce planets with a different overall composition.

Methods. We compiled abundances for Fe, O, C, Mg, and Si in a large sample of solar neighbourhood stars that belong to different galactic populations. We then used a simple stoichiometric model to predict the expected iron-to-silicate mass fraction and water mass fraction of the planet building blocks, as well as the summed mass percentage of all heavy elements in the disc.

Results. Assuming that overall the chemical composition of the planet building blocks will be reflected in the composition of the formed planets, we show that according to our model, discs around stars from different galactic populations, as well as around stars from different regions in the Galaxy, are expected to form rocky planets with significantly different iron-to-silicate mass fractions. The available water mass fraction also changes significantly from one galactic population to another.

Conclusions. The results may be used to set constraints for models of planet formation and chemical composition. Furthermore, the results may have impact on our understanding of the frequency of planets in the Galaxy, as well as on the existence of conditions for habitability.